Referring to FIG. 1, there is shown a schematic block diagram of a radio assembly 100. Radio assembly 100 includes a conventional radio 104 such as a two-way frequency-modulated (FM) radio. Radio 104 is coupled to a battery pack or battery assembly 106 via B+line 116 and B- or ground potential line 114. In a typical design, battery assembly 106 is mechanically connected to radio 104. Electrical contacts on radio 104 and battery assembly 106 provided for the electrical interconnection of radio 104 to battery assembly 106, once battery assembly 106 is connected to radio 104.
Battery assembly 106 includes a plurality of battery cells 108 coupled in series. Battery cells 108 are typically rechargeable and can be of many different varieties such as Nickel-Cadmium, Nickel-Metal-Hydride, etc. Battery cells 108 can also be packaged in a number of different geometry's, such as by using cylindrical cells, prismatic cells, etc. Battery assembly 106 as shown also includes a capacity resistor (Rc) 106 as known in the art which informs an external battery charger 102, charging battery assembly 106, the charge capacity of assembly 106 (i.e., a particular resistance value could inform the charger that it is charging a 1000 milli-ampere-hour (mall) capacity battery). Capacity resistor 110 helps the external charger determine the amount of charge current to provide battery assembly 106. A thermistor (Rt) 112 is also utilized in the assembly in order to track the temperature of battery assembly 106 when it is being charged. A set of battery contacts 120, 122, 124 and 126 provide for electrical interconnection to external battery charger 102 whenever battery assembly 106 requires charging. Finally, a protection diode 118 is also included for blocking any current from flowing from battery cells 108 into the external charger.
Charger 102 consists of a charger monitor circuit 128, which can consist of a well known microprocessor or microcontroller as known in the art and appropriate control software. Charger monitor circuit 128 controls charger control circuit 130 which provides current to battery 106 in order to charge the battery. A control signal is transmitted by charger monitor circuit 128 to charger control circuit 130 via bus 140, the control signal informs charger control circuit 130 on how much current to source via line 129 to battery 106.
Charger monitor circuit 128 contains three analog to digital (A/D) ports 132, 134 and 136. A/D port 132 monitors the voltage on the B+line. A/D port 134 senses the resistance of capacity resistor Rc 110 and A/D port 136 in turn senses the resistance of thermistor Rt 112, as its resistance changes once the battery begins charging. A/D ports 134 and 136 include external pull-up resistors which are used to determine the resistance's of Rc 110 and Rt 112 indirectly, by determining the voltage level at A/D ports 134 and 136, respectively. The voltage level at ports 134 and 136 change with changes in capacity resistor 110 and thermistor 112 respectively.
Typically, the discrete components (e.g., thermistor 112, diode, 118 and capacity resistor 110, etc.)of battery assembly 106 are interconnected to each other using a flexible circuit board as known in the art. The flexible circuit will also help in the interconnection of battery cells 108. Thermistor 112 is typically electrically soldered to the flexible circuit and the flexible circuit is then placed in close proximity to battery cells 108 which help form battery assembly 106. With the move in the electronics industry of providing even smaller assemblies, newer surface mount thermistors are beginning to be employed in the manufacture of battery assemblies and other heat producing assemblies.
A major problem arises when using present day thermistors because of their small size, one finds that the temperature measurement capability of thermistors when employed in heat producing assemblies to measure temperature changes, is confined to the sensing of temperature changes in a small area, or "spot", of the overall assembly. This is especially true in battery assemblies where the change in temperature of the battery cells rise quickly due to the rapid charging of the battery cells. For example, referring to FIG. 2, a top view of a prior art battery assembly 200 is shown. Battery assembly 200 includes a battery housing 204 and a plurality of cylindrically shaped battery cells 202. Battery cells 202 are electrically interconnected to each other in series, with the positive terminal of the battery group connected to contact 208 located on flexible circuit 212, while the negative terminal of the battery group is coupled to contact 210. A thermistor 206 is provided on flexible circuit 212, but as can be seen, it is in thermal proximity to only one of the battery cells 202. This causes thermistor 206 to only accurately sense changes in temperature of the battery cell it is closest to.
When using prismatic cell geometries, this spot temperature sensing problem becomes even more pronounced, due to the large surface area of prismatic cell packages. In the event that the battery cell which the thermistor is closest to is damaged, and is no longer taking on charge, the thermistor will be informing the battery charger that the battery assembly is at a normal temperature, when in fact the other battery cells may be overheating. A need thus exists for providing an improvement in the thermal sensing of electronic assemblies such as battery assemblies.